See also https://perldoc.perl.org/perl5360delta#Variable-length-lookbehind-is-mostly-no-longer-considered-experimental

I do wish it could work in even more cases, like where the beginning point is known (e.g. (?<=\A...) or with \G) or something like (?<=...\1...) where the length is known when running that part of the regex even if not when compiling it.